1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method that form an image on a print medium by ejecting ink droplets (hereinafter referred to as ink) onto the print medium from a print head.
2. Description of the Related Art
The ink jet printing method has many advantages, such as low noise, low running cost and a relative ease with which the apparatus can be reduced in size and upgraded to have a color printing capability. As digital input devices have achieved technical advances and come into wide use in recent years, there are growing calls in an ink jet printing apparatus market for a high-definition, photo-quality image output. To cope with this demand, efforts are being made to reduce the volume of ink droplets ejected from the print head in the ink jet printing apparatus of recent years.
To realize a high image preservation required for a photograph quality, ink jet printing apparatus that use pigment ink are growing in number year by year. Pigments have higher color saturation than dyes, are not easily affected by ozone and ultraviolet rays and also have higher water-fastness. Stable pigments, both physically and chemically, are highly valuable for use in the ink jet printing apparatus. However, since pigments are not as easily soluble as dyes, appropriate dispersion processing or technique is required for uniformly dispersing pigment particles in a solvent and keeping them in the dispersed state.
For stable dispersion of pigment particles in ink, it is generally practiced to add surfactant or polymer dispersant in ink. These additives comprise a hydrophobic part that adsorbs on the surface of pigment particles and a hydrophilic part that spreads into water producing a three-dimensional and electrostatic dispersion stability. They provide a variety of functions depending on their kind and combination. Thus, by optimally controlling the composition of additives, it is possible to realize ink that has a stable dispersion state and assures high reliability of printing operation.
Optimal composition and amount of additives added to ink vary according to the ink color, i.e., the kind and density of pigment used. Thus, in the color ink jet printing apparatus using a plurality of color pigment inks, the physical property often differs from one ink to another.
Focusing on the fact that such physical property variations lead to instability of operations, such as suction-based recovery operation in the apparatus, Japanese Patent Laid-Open No. 2003-176431 discloses a technique for keeping the physical properties of a plurality of inks composed of different pigments within a predetermined range.
However, as the ink droplets are being progressively reduced in size in recent years, some instances have been recognized in which differences in physical property among different color inks have come to affect the ink ejection operation and even the image quality.
FIG. 1 shows how ink ejections are affected by physical property differences. In the figure, a printing element board 24 on a print head 1 has a plurality of nozzle columns arranged in a main scan direction, each adapted to eject a different ink. Here is shown a state in which different inks are being ejected from different nozzle columns when the nozzle columns are driven under the same condition.
Generally, an ink droplet ejected from a nozzle separates into a main drop 101 that constitutes a major part of the ejected volume, and a small satellite 102. At this time, if the ejection drive conditions are the same, a flying speed of the main drop 101 is almost constant even among different ink colors. Studies conducted by the inventor of this invention, however, have found that the speed of the satellite 102, which has a small mass, varies depending on the physical property of the ink, particularly a viscosity. In FIG. 1, a speed difference among different satellites 102 is shown to have translated into a difference among the inks in a distance between the main drop 101 and its satellite 102.
In a serial type ink jet printing apparatus that forms an image by reciprocally scanning a carriage mounting the print head over the print medium, the distance between the main drop and its satellite translates into a deviation of the landing position on the print medium in the main scan direction. So, when, as shown in FIG. 1, the distance between the main drop 101 and its satellite 102 differs among different colors, the amount of landing position deviation varies among different colors. The landing position deviation between the main drop and satellite deforms the shape of a dot formed on the print medium, enlarging its area. Therefore, the greater the distance between the main drop and its satellite, i.e., the slower the flying speed of the satellite, the larger the area of the dot formed on the print medium will be. As a result, in an ink jet printing apparatus that forms a color image using a plurality of inks with different physical properties, the dot landing deviations cause image density variations and color deviations, degrading the printed image. Particularly in a high-speed print mode that moves the carriage at high speed while ejecting ink, the distance between the main drop and its satellite increases, making the density difference among ink colors more conspicuous.
A study conducted by the inventor of this invention has found that speed variations among satellites are caused mainly by differences in ink viscosity. Our comprehensive experiments have observed that, under the condition of the same ejection speeds and the same ejection volumes (of main drops), the length of a liquid column while flying (the distance from the main drop to the satellite) increases as the viscosity increases and that the number of satellites tends to decrease as the surface tension increases.
As described above, in an ink jet printing apparatus of recent years that forms an image by using small drops of color inks, differences in physical property among inks translate into differences in the ejection characteristic, which in turn degrades an image quality. Such an image problem caused by the ejection speed difference between the main drop and its satellite has newly been brought to the fore by the rapid size reduction of ink droplets in recent years. This is because as the main drop becomes smaller, the presence of the satellite becomes more significant, making the printing position deviations of these drops more likely to affect the image being printed. Japanese Patent Laid-Open No. 2003-176431 does not refer at all to the image deterioration problem mentioned above, though it pays attention to the fact that differences in physical property among different ink colors affect a suction operation. So, even in a case where a plurality of inks used have physical properties that fall within the range of conditions disclosed in Japanese Patent Laid-Open No. 2003-176431, the difference in the flying speed among the satellites still results. It therefore has not been possible to prevent degradations in image quality that the present invention aims to solve.